1. Field of the Invention
This invention relates generally to a gyro apparatus and, more particularly, is directed to a vibration-type gyro apparatus using a vibrating mass portion.
2. Description of the Prior Art
A summary of a vibration-type gyro apparatus previously proposed (disclosed in Japanese patent application No. 58-45234) by the same assignee of the present application will be described with reference to FIGS. 1 and 2. FIG. 1 is a perspective view of such previously proposed vibration-type gyro apparatus and FIG. 2 is a side view thereof.
Referring initially to FIG. 1, this gyro apparatus has a base table 2 of a flat plate-shape on which a detection piezo-electric element 30 with a thin plate-shape made of a rectangular bimorph leaf is attached so as to be substantially perpendicular to the upper surface of the base table 2 so as to detect an input angular velocity .OMEGA.. When the detection piezo-electric element 30 is attached on the base table 2, an attaching portion 30A may be used as required. Reference numeral 1 designates generally a tuning fork which consists of a pair of vibration mass portions 1-1 each having a large mass, a pair of flexible portions 1-2 respectively coupled to the vibration mass portions 1-1 and a base portion 1-3 for connecting the free ends of the both flexible portions 1-2 and 1-2. On the upper surface of the base portion 1-3, there is fixed an L-shaped attaching portion 1-4 such that one leg portion 1-4a thereof extends upward substantially in the vertical direction and that the other leg portion 1-4b thereof extends in the direction substantially parallel to the both flexible portions 1-2 and 1-2. A counter weight portion 1-5 is attached to the under surface of the base portion 1-3.
The tuning fork 1 formed as above is fixed to the vibration detection piezo-electric element 30 of thin plate-shape in the following manner. That is, the leg portion 1-4b of the L-shaped attaching portion 1-4 is fixed to the upper portion of the piezo-electric element 30 so that the width direction B of the piezo-electric element 30 of thin plate-shape is extended in the spacing or gap g between the both flexible portions 1-2 and 1-2 of the tuning fork 1. Consequently, as shown in FIG. 2 that is a side view of FIG. 1, the tuning fork 1 is attached to the piezo-electric element 30 such that the vibration plane (tuning fork surface) of the tuning fork 1 becomes substantially parallel to the plane of the base table 2 located horizontally, or the vibration plane thereof becomes perpendicular to the longitudinal central axis X-X of the piezo-electric element 30. In this case, the spacing or gap g between the flexible portions 1-2 and 1-2 is designed in such size and shape that even when the piezo-electric element 30 is vibrated to incline the vibration surface of the tuning fork 1, the piezo-electric element 30 and the both flexible portions 1-2, 1-2 can be prevented from contacting with one another. Further, the height of the piezo-electric element 30 on the base table 2 is determined so that the vibration mass portions 1-1 of the tuning fork 1 and the counter weight portion 1-5, etc. can be prevented from contacting with the upper surface of the base table 2 even when the vibration plane of the tuning fork 1 is inclined.
In FIG. 1, reference numerals 4, 4 designate drive piezo-electric elements that are respectively attached to the both flexible portions 1-2, 1-2. These drive piezo-electric elements 4, 4 are driven by, for example, a signal from an AC signal source 5, whereby to excite the vibration mass portions 1-1, 1-1 of the tuning fork 1 to alternately vibrate at a velocity, v.
If under this state the input angular velocity .OMEGA. is applied around a tuning fork axis indicated by Z-Z in FIG. 1, a Coriolis force Fc proportional to the product of the velocity v and the angular velocity .OMEGA. occurs at each of the two vibration mass portions 1-1 in the opposite directions to thereby alternately vibrate the tuning fork 1 around the above mentioned tuning fork axis Z-Z at the frequency same as that of the tuning fork 1. The displacement angle of this alternate vibration is converted to an electrical signal by the detection piezo-electric element 30 of bimorph-leaf type which is delivered as an output voltage.
In this case, when the output voltage from the detection piezo-electric element 30 is supplied to a demodulator 7 together with the signal from the AC signal source 5 as the reference voltage and is synchronously rectified, the demodulator 7 produces a voltage proportional to the angular velocity .OMEGA. applied around the tuning fork axis Z-Z of the tuning fork 1 perpendicular to the axis X-X, thus the vibration-type gyro apparatus being constructed.
FIG. 3 is a functional block diagram of the vibration-type gyro apparatus shown in FIGS. 1 and 2. Let us now assume that the product of the mass of the both vibration mass portions 1-1 of the tuning fork 1, the vibration amplitude and the vibration frequency, etc of the vibration mass portions 1-1 be expressed by a proportional constant K.sub.T. Then, an alternate torque .OMEGA.K.sub.T sin .omega..sub.t made by the Coriolis force Fc provided by multiplying the angular velocity .OMEGA. around the tuning fork axis Z-Z and the proportional constant K.sub.T urges the whole of the tuning fork 1 to alternately angular-vibrate around the tuning fork axis Z-Z. In FIG. 3, reference numeral 31 designates a mechanical system including the tuning fork 1 provided around the axis Z-Z and the expression within the block 31 represents the transfer function thereof. A displacement angle .theta. of the alternate angular vibration is converted to an electrical signal by the piezo-electric element 30 and amplified in AC by a pre-amplifier 35. Thereafter, it is supplied to the demodulator 7 and then synchronously rectified with the signal from the AC signal source 5. The output from the demodulator 7 is then fed to a filter 36. Thus, from the filter 36 there is produced a voltage Y proportional to the angular velocity .OMEGA.. In FIG. 3, reference letter Kv designates a deviation angle-voltage conversion constant of the piezo-electric element 30 and K.sub.1 designates the gain of the pre-amplifier 35. In the transfer function provided within the block 31, reference letter I denotes inertial efficiency of the tuning fork system around the tuning fork axis Z-Z, C.sub.1 denotes an equivalent viscosity resistance coefficient of the tuning fork system, K denotes a torque spring constant of the piezo-electric element 30 around the tuning fork axis Z-Z and S denotes a Laplace operator, respectively.
However, such previously proposed vibration-type gyro apparatus is designed to detect the vibration amplitude of the tuning fork 1 by the piezo-electric element 30, and if the ambient temperature is changed, the output voltage is changed because the temperature sensitivity of the above piezo-electric element 30 is large with the result that the vibration-type gyro apparatus has high (poor) temperature sensitivity.
Another example of the prior art vibration-type gyro apparatus will be described with reference to FIG. 4. In this prior art vibration-type gyro apparatus of FIG. 4, a tuning fork 101 is supported on a base table 102 through a flexible shaft 103. The central line of the flexible shaft 103 is made coincident with the central line of both leg portions 111 and 111A of the tuning fork 101, so that this central line is referred to as an axis Z-Z hereinafter. Reference numerals 104 and 104A respectively designate drive apparatus which are adapted to vibrate the both leg portions 111 and 111A of the tuning fork 101. Various apparatus, such as an electrostriction element, an electromagnetic forcer and so on can be used as the drive apparatus 104 and 104A. When an AC drive voltage from an AC voltage source 105 is supplied to the drive apparatus 104 and 104A to vibrate the tuning fork 101, both leg portions 111 and 111A of the tuning fork 101 are vibrated symmetrically with respect to the axis Z-Z. Let us consider a certain moment. At this time, as shown in FIG. 4, if the tip ends of both leg portions 111 and 111A of the tuning fork 101 are being moved to the outside at a velocity, v and the whole of the vibration-type gyro apparatus is being constantly rotated together with the base table 102 around the axis Z-Z at an angular velocity .OMEGA., Coriolis force Fc occurs at one leg portion 111 of the tuning fork 101 and Coriolis force F.sub.C1 in parallel to but opposite in direction to the former occurs in the other leg portion 111A so that the tuning fork 101 is urged to twist the flexible shaft 103 relative to the base table 102. This action is caused by the couple of force made of Coriolis forces F.sub.C and F.sub.C1. Since the tuning fork 101 is vibrated, when the movements of the both leg portions 111 and 111A are oriented toward the inside opposite to the former state and the velocities v thereof are reversed in direction opposite to that shown in FIG. 4, the Coriolis forces F.sub.C and F.sub.C1 are also reversed in direction so that the couple of force made by the Coriolis forces F.sub.C and F.sub.C1 is reversed in direction. Therefore, if a constant angular velocity .OMEGA. exists around the axis Z-Z, the tuning fork 101 produces a twisting vibration around the axis Z-Z for the base table 102. In this case, the amplitude thereof is in proportion to the couple of force made by the Coriolis forces F.sub.C and F.sub.C1 so that it becomes proportional to the angular velocity .OMEGA.. Accordingly, if a twist detector 108 for detecting the twisting vibration of the tuning fork 101 around the axis Z-Z is provided at the base portion of the tuning fork 101 as shown in FIG. 4 and an output 107 therefrom is detected, the angular velocity .OMEGA. can be known. Thus, the vibration-type gyro apparatus of FIG. 4 can be used equivalently to a rate gyro as the detector for detecting the angular velocity .OMEGA. around the axis Z-Z.
As shown in FIG. 4, the prior art normal exciting method for the tuning fork 101 comprises a structure in which the AC voltage of a constant frequency and a constant voltage is applied to the drive apparatus 104 and 104A of the tuning fork 101. If it is not required to vibrate the tuning fork 101 at its dynamical resonant frequency, this method is simple and to the point. However, if the resonant frequency of the tuning fork 101 is not used, it is very difficult to produce a large amplitude, and a power efficiency thereof is not so good. For this reason, it is strongly suggested to vibrate the tuning fork 101 at its resonant frequency in use. To this end, a power source frequency is made coincident with the dynamical resonant frequency of the tuning fork and the power source is adjusted so as to produce a voltage which can provide a necessary amplitude.
However, this method has a serious defect. That is, since the dynamical resonant frequency of this kind of dynamic system is extremely sharp and the resonant frequency changes because it is sensitive to temperature, even if a satisfactory resonant state is established at a certain time, the dynamical resonant frequency is gradually displaced from a constant electrical frequency due to the change of the ambient temperature and so on. As a result, the amplitude rapidly decreases and the characteristic of the vibration-type gyro apparatus is suddenly deteriorated. Therefore, the prior art vibration-type gyro apparatus has defects that the vibration thereof is easily affected by the ambient temperature and the like, the amplitude thereof is greatly changed and the sensitivity thereof is not constant. Further, the power source for producing a constant frequency and a constant voltage is not so cheap. Accordingly, if the accuracy of the vibration-type gyro apparatus is increased, an expensive power source has to be used. In this case, however, in spite of the expensive power source, the prior art vibration-type gyro apparatus can not increase satisfactorily the efficiency as is required by the vibration-type gyro apparatus because of the above mentioned defects.
Furthermore, the above dynamical resonant frequency is changed a little with the change of ambient temperature or the like. In this case, only if the amplitude of the vibration of the tuning fork 101 is held constant, the velocity v of the leg portions 111, 111A of the tuning fork 101 would not be made constant (the velocity v of the tuning fork 101 is as given by the product of vibration amplitude and vibration frequency). This becomes a scale factor error when the gyro apparatus is used as the vibration-type gyro apparatus.